I. Field of the Invention
This invention relates generally to a method and apparatus for optically measuring a distance; and more particularly, to a method and apparatus for optically measuring the distance between a patient's cornea and the outer edge of his eyeglass lenses.
II. Prior Art and other Considerations
In a conventional method for determining the distance between the patient's cornea and his eyeglass lenses the distance is estimated by hand using a ruler. This method has the disadvantages of a loss of accuracy and a difficulty of use.
Another prior art method uses an inside caliper to measure the distance from the patient's cornea to the lenses of his eyeglasses. This method has the disadvantage that the caliper comes into physical contact with the patient's eyelids. This method also cannot guarantee that the distance is being measured from the proper point on the cornea.
Heretofore, an optical measuring instrument for example as the projector has been of such an arrangement that the object being measured is rested on a mount and illuminated by parallel ray beams, an image of projection of the object being measured is made to focus on a screen by a transmitted light or a reflected light of the parallel ray beams, and the dimensions, contour and the like of the object being measured. However, in general, so-called blurs are found at the edges of the image of the object being measured, which has been projected on the screen. In consequence, it is difficult to accurately read a measured value from the coincidence between the image formed on the screen and a hair line.
Another projection method employs the science of interferences. However, this method presents the disadvantage that interferences such as external irregular lights affect the performance of the projector to a great extent and the measuring accuracy is greatly deteriorated due to variations in the signal obtained from the photoelectric element and in the reference voltage.
Still another projection method uses a photoelectric element. However, this method presents the disadvantages that the position of the edge thus detected may differ depending on the value of the relative movement rates between the photoelectric element and the image of projection, and further, the measuring accuracy is greatly affected by a variation in a reference voltage.
Further, the scope of application of the illumination ray beams to light intensity is small, and a sensor portion or a circuit portion becomes complicated in construction.
Particularly, in the projector, the brightness of the image projected on the screen is varied due to the fatigue of a light source lamp for illumination, the characteristics of lenses in a projection system and the external irregular lights.
Furthermore, according to the conventional edge detecting methods, when the focus of the image of projection is shifted, the wave forms emitted from the photoelectric element or elements become gently sloped, thereby presenting the disadvantage that no accurate edge can be detected. The disadvantage is common in the edge detection among the optical measuring instruments wherein, in general, a transmitted light or a reflected light is detected to directly or indirectly measure dimensions and the like of an object being measured.
It is known in the art to provide optical measuring apparatus for measuring the diameter of large work pieces, comprising a pair of telescopic sights displaceable on a common guide and having a pair of measuring marks which can be aimed at the two end points of the diameter of the work piece. The displacement of the telescopic sights can be read on a scale, so as to indicate the length of the diameter. This measuring apparatus can be used, however, only for measuring the diameter of stationary work pieces, but cannot be used if the work piece is moving, for example rotating, because then the reference marks are removed from the aiming position.
According to another apparatus, known in the art, a light ray is emitted towards the work piece, comes into contact with the surface of the latter and is viewed in a microscope. Interference stripes are thus produced, the positions of which can be determined by means of a reference mark. Thereafter, the apparatus is displaced until the reference mark on the other end of the work piece receives the same interference strips. The displacement indicates the desired value of the diameter. This apparatus suffers from the disadvantage of being complicated and time-consuming in operation and cannot be used for effecting the measurement of work pieces having specular of semi-specular surfaces.
Still another apparatus has, in addition to a pair of telescopic sights displaceable on a guide, and a scale from which the displacement of the telescopic sights can be read, a reference mark and projecting means for projecting said reference mark in the direction of the diameter of the work piece onto the two end points of the diameter of the work piece. The reference marks as projected can be viewed with the two telescopic sights. In case of a rotating work piece and position of the two reference marks is not changed as long as the diameter remains the same. Preferably, light bands are used as reference marks and a diaphragm is provided, allowing for the variation of the respective widths and light intensities of the light bands. This is still relatively complicated and time-consuming in operation.